with a report of the battle. So, the officer relates the brutal slaying of the rebel Macdonwald by the braveMacbeth. As he listens, the king seems an observer, and certainly not a participant, of the battle; and then he praises Macbeth, "O valiant cousin! Worthy gentleman!" (1.2.34)

• He displays an emotionalism that is unbecoming of a king

When Duncan announces his heir, he becomes so effusive that he can hardly say the words:

My plenteous joys,
Wanton in fullness, seek to hide themselves
In drops of sorrow. (1.4.39-41)

• He is unable to know what others are thinking, and unaware of the dangers of a setting.

As King Duncan approaches the castle of Macbeth, he is unable to sense any danger from the ruthless warrior on whom he has bestowed the title of Thane of Cawdor. Instead, he feels everything is lovely:

This castle hath a pleasant seat; the air
Nimbly and sweetly recommends itself
Unto our gentle senses (1.6.1-3)

Also unable to detect any danger from Lady Macbeth, who has just finished asking the preternatural spirits to unsex her, King Duncan addresses her with naïveté, saying that although his visit may be inconvenient, she should ask God to reward him for coming because his love prompted his visit.

See, see, our honored hostess!
The love that follows us sometime is our trouble,
Which still we thank as love. Herein I teach you
How you shall bid God 'ield us for your pains
And thank you for your trouble. (1.6.10-14)

Further, he misjudges both Macbeth and Lady Macbeth:

And his great love, sharp as his spur, hath help him

To his home before us. Fair and noble hostess,

We are your guest tonight. (1.6.23-25)

• Duncan is too meek in his position as king

In his soliloquy in Scene 7 of Act I, Macbeth reflects upon his relative, King Duncan, who is so meek and virtuous that his murder will "plead like angels" against his "horrid deed." 

...his virtues
Will plead like angels trumpet-tongued against
The deep damnation of his taking-off (1.7.18-20)

Lady Macbeth also recognizes Duncan's meekness as she tells her husband that she could have done the deed if Duncan had not reminded her of her father:

                 ...Had he not resembled
My father as he slept, I had done't (12.2.12-13)

Certainly, Shakespearean scholars, such as Janet Adelman, have alluded to King Duncan's nurturing nature, his "womanish softness," and his child-like trust and inability to interpret what is in people's faces. He is a good man, but too weak for the world in which he lives. 

Although Duncan is depicted as a benevolent, compassionate king, he is a poor judge of character and too trusting. At the beginning of the play, the Thane of Cawdor takes advantage of Duncan's trust by joining forces with the King of Norway. Fortunately, Macbeth and Banquo are able to defeat the Norwegian forces and the Thane of Cawdor is captured and executed.

In act 1, scene 4, King Duncan comments on his inability to recognize a person's true intentions by saying,

There’s no art
To find the mind’s construction in the face.
He was a gentleman on whom I built
An absolute trust. (1.4.13–16).

King Duncan openly admits that he is a poor judge of character by placing his "absolute trust" in a traitor. King Duncan's inability to recognize the Thane of Cawdor as a dangerous enemy foreshadows his mistake in trusting Macbeth.

After Macbeth is given the title Thane of Cawdor and the first prophecy is confirmed, he entertains the idea of murdering King Duncan and begins planning the assassination with his ambitious wife. Unfortunately, King Duncan once again places his trust in the wrong people. The audience recognizes that Macbeth and his wife are Duncan's worst enemies and currently plotting his demise. When King Duncan arrives at Macbeth's castle, he demonstrates his poor judge of character by referring to Lady Macbeth as a "fair and noble hostess" and is excited about the possibility of visiting their estate. Later that night, Lady Macbeth drugs Duncan's chamberlains and her husband assassinates the king.

Overall, King Duncan's major shortcoming is his poor judge of character. King Duncan is entirely too trusting and does not recognize his true enemies.

In act 1, scene 4, King Duncan explains to Malcolm that appearances are often deceiving and that he regretted putting an "absolute trust" in the former Thane of Cawdor, who has recently been executed for treason. Ironically, Duncan fails to recognize that Macbeth, the new Thane of Cawdor, presents an even greater threat.

Duncan trusts Macbeth, calling him "valiant" and "noble." Since Macbeth has recently proven loyal in battle, Duncan mistakenly believes that "brave Macbeth" embodies a devotion toward him which cannot be shaken. He thus visits Macbeth's castle without any reservations, believing that the setting is "pleasant" and "sweet." Lady Macbeth, who has spent much energy convincing Macbeth to move forward with plans to murder the king, greets Duncan; he gushes over her "honor" and proclaims that she is a "fair and noble hostess."

Duncan is characterized as fair and honorable, but he has a history of trusting the wrong people. The former Thane of Cawdor, Macbeth, and Lady Macbeth all prove to be untrustworthy characters, and Duncan repeatedly misjudges the intent of those who wish to cause him harm. His lack of intuition regarding characters do not deserve his trust eventually positions Duncan to be murdered by the man whom he has proclaimed to be a "worthy gentleman."

Malcolm seems like an ineffectual character in many ways. He does not become effectual until almost the end of the play when he returns to Scotland at the head of the English army.

Early in Macbeth, Shakespeare leaves an important question unanswered. After Duncan’s body was discovered, why didn’t Malcolm, his eldest son and heir apparent, stay and claim the crown instead of fleeing to England? Duncan had named Malcolm the Prince of Cumberland and publicly proclaimed him his heir.

Sons, kinsmen, thanes,
And you whose places are the nearest, know
We will establish our estate upon
Our eldest, Malcolm, whom we name hereafter
The Prince of Cumberland

It was very convenient for Macbeth that both Malcolm and his brother Donalbain, next in line of succession, should have decided to flee, because the assassin could blame Duncan’s murder on them and claim the crown himself—but he couldn’t have foreseen this development. Another unanswered question is: Why didn’t Macbeth murder Duncan’s two sons in their sleep on the same night he murdered their father?

In Act 4, Scene 3, Shakespeare seems to be tying up loose ends. In the very strange conversation between Malcolm and Macduff, the younger man accuses himself of all kinds of faults which he feels make him unworthy to be King of Scotland. At one point he says: “…better Macbeth  / Than such an one [as himself] to reign.” All of this confession would seem intended to explain why Malcolm did not stay and claim the crown on the morning of his father’s murder. But then he disclaims all his self-accusations and tells Macduff that he was only testing his loyalty because he suspected him of being an agent of Macbeth. This seems little short of preposterous. Even if he suspected Macduff of being a spy for Macbeth, it was very bad form to say so to a thane who had come there to help him.

The only explanation for the inclusion of this long and unproductive scene is that Shakespeare wanted to explain why Malcolm didn’t claim the crown in the first place. Evidently he felt unworthy to be king. But what is he doing in England trying to raise an army of ten thousand men to invade Scotland? The other apparent purpose of this scene is to bolster Malcolm as a character in the minds of Shakespeare’s audience. He had very little to do or say while he was at Dunsinane, but he has to become Macbeth’s chief antagonist and leader of the invading army. The scene gives the audience an opportunity to familiarize themselves with this character. It also gives the audience further opportunity to become better acquainted with Macduff, who will be the one to deliver Macbeth’s death blow.

Otherwise, the scene seems like a waste of words. Malcolm says he is unworthy to have claimed the crown in place of Macbeth, and then he disclaims every bad thing he said about himself and makes himself appear almost as pure as the saintly English King Edward.

Scene 3 of Act 4 is never included in film adaptations of Macbeth. It is quite sufficient for the viewer to understand that Malcolm has fled to England, raised an army, and come back with Macduff to claim the Scottish throne. The question of why Malcolm fled from Scotland in the first place remains unanswered. The viewer might accept his explanation that he didn't feel worthy to become king because of all his vices if he didn't disclaim all those vices practically in the same breath.